1. Field of the Invention
The invention relates to a printing method for a printer having a printhead with a plurality of print elements and capable of printing a binary pixel image. The invention further relates to a printer and to a computer program implementing this method. The invention is applicable, for example, to an ink jet printer the printhead of which comprises a plurality of nozzles as print elements.
2. Discussion of the Background Art
Typically, the nozzles of an ink jet printer are arranged in a line that extends in parallel with the direction (subscanning direction) in which a recording medium, e.g. paper, is transported through the printer, and the printhead scans the paper in a direction (main scanning direction) perpendicular to the subscanning direction. A complete swath of the image is printed in a single pass of the printhead, and then the paper is transported by the width of the swath so as to print the next swath. When a nozzle of the printhead is defective, e.g. has become clogged, the corresponding pixel line is missing in the printed image, so that image information is lost and the quality of the print is degraded.
A printer may also be operated in a multi-pass mode, in which only part of the image information of a swath is printed in a first pass and the missing pixels are filled-in during one or more subsequent passes of the printhead. In this case, it is sometimes possible that a defective nozzle is backed-up by a non-defective nozzle, although the cost of productivity may increase.
U.S. Pat. No. 6,215,557 is directed to a method of the type indicated above, wherein, when a nozzle is defective, the print data are altered so as to bypass the faulty nozzle. This means that a pixel that would have but cannot be printed with the defective nozzle is substituted by printing an extra pixel in one of the neighbouring lines that are printed with non-defective nozzles, so that the average optical density of the image area is conserved and the defect resulting from the nozzle failure is camouflaged and becomes almost imperceptible. This method involves a specific algorithm that operates on a bitmap, which represents the print data, and shifts each pixel that cannot be printed to a neighbouring pixel position. However, if this neighbouring pixel position happens to be occupied by a pixel already printed, anyway, pursuant to the original print data, then the extra pixel cannot be printed, and a loss of image information will nevertheless occur.